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Abstract:

The invention relates to an EKG measurement device. The EKG measurement
device comprises a number of EKG electrodes and a common-mode measurement
unit connected on its input side to the EKG electrodes. Inventively an
EKG trigger unit is connected on its input side to the EKG electrodes and
to the common-mode measurement unit.

Claims:

1.-9. (canceled)

10. An EKG measurement device, comprising:an EKG electrode that generates
an EKG signal of a patient;a common-mode measurement unit with an input
side connected to the EKG electrode that generates a common mode signal
based on the EKG signal; andan EKG trigger unit with an input side
connected to the EKG electrode and to the common-mode measurement unit
that detects the common mode signal and triggers the EKG measurement
device based on the detection.

11. The EKG measurement device as claimed in claim 10, wherein the EKG
trigger unit comprises a trigger signal generator unit that generates a
trigger signal based on the EKG signal.

12. The EKG measurement device as claimed in claim 1, wherein the EKG
trigger unit comprises a control unit that controls the trigger signal
generation unit based on the common mode signal.

13. The EKG measurement device as claimed in claim 10,wherein the
common-mode measurement unit is connected to the EKG trigger unit via a
subtractor, andwherein an input side of the subtractor is connected to a
further electrode.

14. The EKG measurement device as claimed in claim 10, wherein an output
side of the common-mode measurement unit is connected to a further
electrode.

15. The EKG measurement device as claimed in claim 10,wherein the
common-mode measurement unit is connected to the EKG trigger unit via a
subtractor,wherein an input side of the subtractor is connected to a
further electrode, andwherein an output side of the common-mode
measurement unit is connected to the further electrode.

16. The EKG measurement device as claimed in claim 10, wherein an inverter
is inserted between the common-mode measurement unit and a further
electrode.

17. The EKG measurement device as claimed in claim 10, wherein the EKG
electrode is connected to the common-mode measurement unit via a lowpass.

18. The EKG measurement device as claimed in claim 10, wherein the
common-mode measurement unit is connected to a further electrode via a
lowpass.

[0002]The invention relates to an EKG measurement device with a number of
EKG electrodes and with a common-mode measurement unit connected on its
input side to the EKG electrodes.

BACKGROUND OF THE INVENTION

[0003]An EKG measurement device typically receives an EKG signal with two
EKG electrodes that are connected to inputs of a high-impedance
differential amplifier. If there is the option of connecting more than
two EKG electrodes to the EKG measurement device, typically one of the
EKG electrodes will be used as a common reference point.

[0004]An EKG measurement device of the type mentioned above is disclosed
in the book by Karsten Meyer-Waarden: "Bioelektrische Signale und ihre
Ableitverfahren (Bioelectric signals and their derivation process)",
1985, Schattauer Verlagsgesellschaft, Stuttgart, Germany, on pages 142 to
143. In the EKG measurement device described therein a noise signal with
a common-mode component is reduced at two EKG measurement electrodes with
the so-called reference potential control method. The noise signal is
created by a displacement current which is produced in its turn here by
an electrical power supply network with alternating current. In the
reference potential control method the reference electrode is not at
reference ground or reference potential, but receives a potential
corresponding to the common-mode component of the noise signal. The noise
signal is tapped off at the two EKG electrodes. The common-mode component
of the noise signal is fed, after amplification, impedance conversion and
inversion, to the electrode which determines the reference potential for
the measurement. The displacement current coupled into the body thus does
not flow against the reference ground at a constant reference potential
but flows into a reference point of which the potential is controlled by
the noise voltage. The displacement current is compensated for by an
opposing current in terms of amount and phase.

[0005]EKG devices are used not only for measurement and monitoring of the
heart function but also in medical imaging to create trigger signals.
Information about the heart phase is obtained from the EKG signal during
imaging in order in this way to synchronize the imaging with the heart
activity. With imaging processes requiring a longer period to record the
image in particular high-quality images of the heart or also images of
regions which pulse with the beating of the heart can be created.

[0006]EKG measurement devices are therefore also advantageous for in-situ
recording of EKG signals during an examination of a patient by means of a
magnetic resonance (MR) device. Operation in the magnetic resonance
device however demands a series of measures to make trouble-free
measurement in the environment of the magnetic resonance device possible
at all. It is well known that strong high-frequency fields in the
megahertz range as well as strong gradient fields in the low-frequency
range are used in the magnetic resonance device for imaging. The EKG
measurement may neither be disturbed by the operation of the magnetic
resonance device nor may it disturb the operation of the magnetic
resonance device itself. EKG measurement devices which are MR-compatible
in the sense stated above are available on the market.

[0007]However the problem with such devices remains magnetic fields which
change over time, as are used in the magnetic resonance device as
magnetic gradient fields for location encoding. According to the law of
induction, changes to magnetic fields over time create noise voltages
which are coupled into the EKG signal received by the EKG electrodes as
noise. Movements of the patient during image recording in the static
magnetic field also create noise signals in accordance with the law of
induction, because the effective surface for the coupling-in is changed
by the movement. These types of magnetically-created noise signals
overlay themselves with the EKG signal created by the body and falsify
this signal.

[0008]Recording a magnetic resonance image synchronized with the heartbeat
however basically demands a reliable detection of the R wave in the EKG
signal. The noise signals generated by the switched gradient fields and
also by rapid movements can however be mistakenly interpreted as an R
wave and thus, because of the incorrect triggering that they generate,
lead to a marked deterioration in the image quality. The practice of
investigating the EKG signals in the trigger unit of the EKG measurement
device for problems caused by magnetic fields is known. To this end the
dynamics of the EKG signals are analyzed and evaluated as to whether the
EKG signal involves an R wave to be detected or a fault. Incorrect
triggering is still not excluded if the dynamics of the noise signal
correspond to those of the R wave in the EKG signal.

SUMMARY OF THE INVENTION

[0009]The underlying object of the invention is now to specify an EKG
measurement arrangement which allows reliable detection of magnetic
field-related faults and for which the risk of emitting incorrect trigger
signals is reduced.

[0010]The present object is achieved by the subject matter of claim 1. The
invention is based on the knowledge that magnetic field-related faults
exhibit a large common-mode component in the EKG signal of the individual
EKG electrodes. Accordingly the EKG measurement device comprises a number
of EKG electrodes and a common-mode measurement unit which is connected
on its input side to the EKG electrodes, with the common-mode measurement
unit being connected on its output side to an EKG trigger unit. The
trigger unit creates a trigger signal when it detects the R wave in the
EKG signal. The common-mode signal generated by the common mode
measurement unit in the case of magnetic field-related faults is fed to
the trigger unit. The trigger unit then detects from the common-mode
signal when the magnetic field-related faults are present and can thus
avoid incorrect triggering. To this end the common-mode signal is
detected together with the EKG signals and processed, e.g. subjected like
the EKG signals to differentiation, signal matching, filtering and A/D
conversion and evaluated in the trigger unit. If an appreciable
common-mode signal appears, this means that a limit value has been
undershot or exceeded and it is assumed that similar faults are also
present in the parallel EKG signal. The outputting of the trigger signal
is then for example, as is known in the prior art, blocked for the period
during which the common-mode signal occurs.

[0011]An advantageous embodiment is characterized by common-mode
measurement unit being linked via a subtractor to the EKG trigger unit
and thus by the subtractor being linked on its input side to a further
electrode. This means that the common-mode signal is related to the
reference potential of the further electrode. The reference electrode is
generally the RL electrode.

[0012]In a further, especially advantageous embodiment, the common-mode
measurement unit is connected on its output side to a further electrode.
A reference potential control, as already described at the outset, is
realized by feedback to a further electrode in order to compensate for
low-frequency faults.

[0013]A further, especially advantageous embodiment, is characterized by
the common-mode measurement unit being connected via a subtractor to the
EKG trigger unit, by the subtractor on its input side being connected to
a further electrode and by the common-mode measurement unit being
connected on its output side to the further electrode. Initially the
feedback largely reduces the low-frequency magnetic field-related noise
signals in the EKG signals. The remaining higher frequency noise signal
is then analyzed in the EKG trigger unit together with the EKG signals in
order to eliminate false trigger signals.

[0014]Further embodiments are characterized by the other subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]The invention is explained below on the basis of six figures. These
are as follows:

[0016]FIG. 1 a block diagram of the structure of a first EKG measurement
device,

[0017]FIG. 2 a block diagram of the structure of a second EKG measurement
device,

[0018]FIG. 3 a block diagram of the structure of a third EKG measurement
device,

[0019]FIG. 4 a block diagram of the structure of a fourth EKG measurement
device,

[0020]FIG. 5 to illustrate the timing of a disrupted EKG measurement
signal as well as the common-mode signal detected simultaneously as a
noise signal, and

[0021]FIG. 6 an overview diagram of a diagnostic magnetic resonance device
with an integrated trigger unit.

DETAILED DESCRIPTION OF THE INVENTION

[0022]The first EKG measurement device shown in the block diagram in FIG.
1 comprises three electrodes 2, 4 and 6, which are stuck onto the surface
of a patient's skin in accordance with a standard color coding in the
traffic light scheme. Thus electrode 2 for example is stuck onto the
right arm with the color coding red, electrode 4 to the left arm with the
color coding yellow and electrode 6 to the left leg with the color coding
green. Over time different so-called derivative schemes have developed.
Particular reference is made here to the derivative scheme in accordance
with Einthoven. In bipolar derivation according to Einthoven the
electrical potential change is measured between the extremities. In this
case Einthoven I stands for the potential difference between the left arm
and the right arm, Einthoven II for the potential difference between the
left leg and the right arm and Einthoven III for the potential difference
between the left leg and the left arm. For examinations in the magnetic
resonance the restriction applies that in general the electrodes 2, 4, 6
are not attached to the extremities themselves but to the thorax. It is
also usual in this connection to attach four electrodes to the upper body
at the corners of a rectangle around the heart. The transfer of the
Einthoven derivative to the thorax is also referred to as chest wall
derivative according to Nehb.

[0023]The electrodes 2, 4, 6 are connected via lowpass filters 7 with a
limit frequency of appr. 130 Hz to a common-mode measurement unit 8 and
to a trigger unit 10. The lowpass filters 7 block the high-frequency
components from the EKG measurement signal which, because of
user-specific high-frequency faults, are overlaid onto the
electrophysiological signal.

[0024]The common-mode measurement unit 8 is constructed in two stages. The
first stage comprises three mean value generators 12, the inputs of which
are connected to two different electrodes 2, 4, 6 in each case. The mean
value generators 12 create an arithmetic mean from the EKG measurement
signals fed to their inputs. They thus include analog summators with an
amplification factor of 0.5. The mean value signals created by the mean
value generators 12 are fed in a second stage to a further mean value
generator 14. The mean value generator 14 forms the arithmetic mean value
of the mean value signals output by mean value generators 12, so that the
mean value of all EKG signals and thereby the common-mode component of
the EKG signals measured by the EKG electrodes are available as the
output signals. A further signal amplification and where necessary also
an impedance matching is undertaken in an amplifier stage 16 downstream
from the mean value generator 14. Depending on the components used, the
mean value generators 12 and 14 can also contain a signal amplifier
stage.

[0025]Like the EKG signals, the common-mode signal output at the output of
the amplifier 16 is fed to the EKG trigger unit 10. The EKG trigger unit
comprises a trigger signal generator unit 10A and a control unit 10B.
After reliable detection of the R wave the trigger signal generator unit
10A creates a trigger pulse in the EKG signal at an output 17. In the
control unit 10B the dynamics and if necessary also the amplitude of the
common-mode signal delivered by the amplifier 16 are analyzed and
compared to reference values. If the two variables exceed specific limit
values which are derived from the reference values, a message is output
for example and the issuing of a trigger signal by the trigger signal
generator unit 10A at output 17 is suppressed.

[0026]FIG. 2 shows in a block diagram of a second embodiment of the EKG
measurement device, which differs from the EKG measurement device shown
in FIG. 1 in that a further electrode 18 is provided as a reference
electrode. The reference electrode 18, when a total of 4 electrodes are
placed in a rectangle around the heart, is to be placed at the corner
closest to the shoulder.

[0027]The reference electrode 18 is connected via a lowpass 7 with a limit
frequency of appr. 130 Hz to the control unit 10B and to a minus input of
a subtractor 20. The common-mode signal from the common-mode measurement
unit 8 is fed to the plus input of the subtractor 20. This signal
processing means that the common-mode signal of the EKG electrodes 2, 4,
6 is merely fed as a difference potential to reference electrode 18 of
the trigger unit 10. This creates an advantageous reference to the body
potential.

[0028]The further processing and evaluation in the EKG trigger unit 10 is
undertaken in the same manner as has already been described with
reference to FIG. 1.

[0029]FIG. 3 shows a further EKG measurement arrangement, which likewise
proceeds from the EKG measurement arrangement depicted in FIG. 1. Here
too the reference electrode 18 is connected via the lowpass filter 7 to
the control unit 10A. The common-mode signal generated by the common-mode
measurement unit 8 is in this case additionally fed via a lowpass filter
22 and an inverter 24 via the lowpass filter 7 to the reference electrode
18. The lowpass filter 22 is designed to feed back low-frequency faults
to allow compensation processes in the body. This external feedback means
that magnetic symmetrical coupled-in low-frequency interference signals
are already compensated for in the EKG signal at all EKG electrodes 2, 4
and 6. Remaining high-frequency residual noise signals are then, as
already described with reference to FIG. 1, analyzed and further
processed in the EKG trigger unit 10.

[0030]The fourth exemplary embodiment of an inventive EKG measurement
device shown in FIG. 4 comprises both feeding back, as described in FIG.
3, and also the differentiation of the common-mode signal from the signal
received by the reference electrode 18 described with reference to FIG.
2. Here too the reference electrode 18 is connected via the lowpass
filter 7 to the control unit 10A. The lowpass 7 blocks MR-specific
high-frequency faults.

[0031]To illustrate the functioning of the EKG measurement device
described here, FIG. 5 shows the signal waveform of an EKG-signal 30 over
time, as generated as a phase difference signal for example from the EKG
signals tapped off from electrodes 2 and 4. In EKG signal 30 an R wave
30A can clearly be seen, after which a trigger pulse 32 is created. In
the further course 30B of the EKG signal 30 however a magnetically
coupled-in fault occurs which no longer allows the R wave to be securely
detected. This fault is clearly shown in the common-mode signal 34. As
soon as the common-mode signal 34 has a specific amplitude and dynamic,
the creation of trigger signals 32 is blocked. After the common-mode
signal 34 decays and after a specific wait time if necessary, trigger
signal generation is enabled again in area 34A.

[0032]FIG. 6 shows in an overview diagram a diagnostic magnetic resonance
device 40 with an integrated EKG trigger unit 10. There are two
particular advantages associated with such a device. On the one hand the
EKG trigger unit can be implemented as software and can run on a control
processor 42 of the MR device 40. This means that the high computing
power in the MR device 40 is also available for the EKG trigger unit 10.
On the other hand the EKG trigger unit 10 can access MR device parameters
and MR process variables in a simple and fast manner.